Several types of control systems, such as dimming ballasts for gas discharge lamps, include control circuitry that operates in conjunction with an external controller. The control circuitry often includes an interface arrangement, as described in FIG. 1, for providing current to an external controller 18 in accordance with the setting of the controller. Specifically, adjustment of the external controller 18 varies the amount of current that the controller draws from a DC voltage source 30 via a current-sourcing circuit 40; correspondingly, a voltage, V.sub.CONTROL, within the interface circuit varies. V.sub.CONTROL is fed to other control circuitry in order to effect some type of action, such as adjusting the illumination level of a fluorescent lamp.
In a number of dimming ballasts, such as that which is disclosed in U.S. Pat. No. 5,457,360, the dimming control circuitry is designed for low voltage operation (i.e., 0 to 10 volts). The dimming control circuitry is usually damaged when a source of high voltage, such as conventional AC line voltage (e.g., 120 volts, 277 volts, etc.), is inadvertently connected to the low voltage dimming control wires. Application of AC line voltage to the dimming control wires, hereinafter referred to as a high-voltage AC fault, damages or destroys components in the dimming interface circuit, as well as other control circuitry that is connected to the dimming interface circuit.
The destructive consequences of a high voltage fault are particularly costly in installations where the dimming control wires from multiple ballasts (e.g., anywhere from 2 to 50 or more) are connected in parallel. As an example, FIG. 2 describes an installation that employs wire harness connectors. One half of the wire harness connector is miswired such that the AC power wires (i.e., black and white) are placed in the portion of the connector intended for the dimming control wires (i.e., violet and gray), and vice-versa. Consequently, when the two halves of the connector are coupled together, the dimming control wires will be connected to the AC line (e.g., 120 volts AC). The result is damage to the circuitry in each of the parallel connected ballasts. This type of wiring mistake is not uncommon in practice, probably because of the large number of wires and the complexity of the wiring within the lighting fixtures.
There are a number of existing approaches for protecting a dimming ballast from damage due to misapplied high voltage across the dimming control wires. One approach is to place a fuse/diode arrangement in the dimming interface circuit. Such an arrangement provides effective protection of the ballast circuitry, but is undesirable because of the simple fact that a blown fuse requires replacement. Because ballasts are not designed to be field repairable, fuse replacement is generally not an economically viable option. Moreover, even if it were practical, fuse replacement would require accessing the installed ballast, opening the ballast, and so forth.
Another approach involves placing an opto-coupler circuit with a high input impedance between the dimming interface circuit and the dimming control wires. This approach provides the significant benefit of non-destructive protection. However, because opto-couplers are known to suffer from very loose tolerances in their transfer characteristics, this approach may significantly detract from the desired operation of the dimming control circuitry and may introduce an unacceptable degree of variation in the resulting range of dimming that can be provided by the ballast. Moreover, use of this approach necessitates extensive redesign of the dimming control circuitry and/or the external dimming controller.
What is needed, therefore, is an arrangement for protecting control circuitry from externally applied high voltages in a non-destructive manner. A further need exists for a protection arrangement that is capable of being readily incorporated into existing control circuitry with minimal modifications, and that presents little or no degradation in the desired control range. Such an arrangement would represent a considerable advance over the prior art.